T-Mobile USA will begin offering an unlimited calling plan, a move that comes in the wake of Boost Mobile's decision to launch a $50 per month unlimited plan.

The carrier, the fourth largest in the United States, will offer a $50 per month unlimited voice service to customers, beginning with a trial in the San Francisco area. T-Mobile said it was offering the option to customers who had been with the carrier for 22 months or more. It will also offer a $135 rebate for customers who switch their wireless service to T-Mobile.

The move comes as T-Mobile faces increasing uncertainty. The carrier added 621,000 new subscribers in the fourth quarter of 2008, its lowest quarterly addition since the second quarter of 2006. Earlier this month, Matt Carter, the head of Boost Mobile, Sprint Nextel's prepaid unit, said that its $50 per month unlimited voice and data plan was peeling away subscribers from T-Mobile.   

In the wake of the Boost move, analysts worried that the unlimited plan would spark a price war with Tier 1 operators. Additionally, in early February Sprint launched a plan aimed at giving perks to its longtime customers.  

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